The present invention relates to a method of controlling a continuously variable automotive transmission by controlling the transmission ratio based on the difference between a target rotational engine speed which is preset according to an indication of the driver's intention of acceleration or deceleration and a detected rotational engine speed.
There is known a method of controlling continuously variable automotive transmission as disclosed by Japanese Laid-Open Patent Publication No. 55(1980)14312.
In the above control method, the rate of change di/dt of the transmission ratio i per unit time can be given by the equation (1): ##EQU1## where Neo is the target rotational engine speed and the detected rotational engine speed. The detected rotational engine speed Ne is expressed by the following equation (2): EQU Ne=C.multidot.V.multidot.i (2)
where V is the speed of travel of the automobile or the output rotational ending speed, and C is a constant.
Therefore, the rate of change dNe/dt of the detected rotational engine speed Ne is as follows: ##EQU2## Since the automobile speed V does not immediately vary greatly even if the speed is changed while the automobile is running, dV/dt.apprxeq.0, and hence the equation (3) approximates to the following equation (4): ##EQU3## From the equations (2) and (4), we get ##EQU4## From the equations (1) and (5), the following equation results: ##EQU5##
Heretofore, a gain K is constant, and it can be understood from the equation (6) that dNe/dt is proportional to Ne, (Neo-Ne), 1/i.
It is now assumed that the gain K in the equation (6) is established such that dNe/dt will be optimum when the detected engine speed Ne is low. Under this assumption, when the difference (Neo-Ne) does not vary greatly, dNe/dt may become excessively large at the time the detected engine speed Ne is increased. When this happens, the engine speed Ne may abruptly increase or decrease, and the difference (Neo-Ne) may repeatedly alternate between positive and negative values, making transmission operation unstable.
Conversely, it is assumed that the gain K is established such that dNe/dt will be optimum when the detected engine speed Ne is high. Under this assumption, when the difference (Neo-Ne) and the transmission ratio i do not vary greatly, dNe/dt may become excessively small at the time the detected engine speed Ne is reduced, and the response of the transmission may become slow.